1. Field of the Invention
The present invention relates to networking systems, and more particularly, processing out of order frames in a SCSI_FCP environment.
2. Background of the Invention
Storage area networks (“SANs”) are commonly used where plural memory storage devices are made available to various host computing systems. Data in a SAN is typically moved from plural host systems (that include computer systems) to storage systems through various controllers/adapters.
Host systems often communicate with storage systems via a host bus adapter (“HBA”, may also be referred to as a “controller” and/or “adapter”) using the “PCI” bus interface. PCI stands for Peripheral Component Interconnect, a local bus standard that was developed by Intel Corporation®. The PCI standard is incorporated herein by reference in its entirety. Most modern computing systems include a PCI bus in addition to a more general expansion bus. PCI is a 64-bit bus and can run at clock speeds of 33, 66 or 133 MHz.
PCI-X is another standard bus that is compatible with existing PCI cards using the PCI bus. PCI-X improves the data transfer rate of PCI from 132 MBps to as much as 1 gigabits per second. The PCI-X standard was developed by IBM®, Hewlett Packard Corporation® and Compaq Corporation® to increase performance of high bandwidth devices, such as Gigabit Ethernet standard and Fibre Channel Standard, and processors that are part of a cluster.
Various other standard interfaces are also used to move data from host systems to storage devices. Fibre channel is one such standard. Fibre channel (incorporated herein by reference in its entirety) is an American National Standard Institute (ANSI) set of standards, which provides a serial transmission protocol for storage and network protocols such as HIPPI, SCSI, IP, ATM and others. Fibre channel provides an input/output interface to meet the requirements of both channel and network users.
Fiber channel supports three different topologies: point-to-point, arbitrated loop and fiber channel fabric. The point-to-point topology attaches two devices directly. The arbitrated loop topology attaches devices in a loop. The fiber channel fabric topology attaches host systems directly to a fabric, which are then connected to multiple devices. The fiber channel fabric topology allows several media types to be interconnected.
Storage devices in a SAN may be coupled to a storage sub-system (for example a RAID system that may also use a HBA) using the Small Computer Systems Interface (“SCSI”) protocol. The SCSI Fibre Channel Protocol (“SCSI_FCP”) is used for communication between a SCSI device and a system using the Fibre Channel network. SCSI_FCP is a mapping protocol for applying a SCSI command set to Fibre Channel command set. Both SCSI and SCSI_FCP standard protocols are incorporated herein by reference in their entirety.
In a typical SCSI_FCP Exchange, an initiator sends a “read” or “write” command to a target. For a read operation, the target sends the requested data to the initiator. For a write command, the target sends a “Ready to Transfer” command informing the initiator that the target is ready to accept the write data. The initiator then sends the write data to the target. Once the data is transferred, the Exchange enters the response phase. The target then sends a response to the initiator with the status of the operation.
As SANs become larger and complex, frames may not arrive in order or may get dropped. The term “in-order” means that frames arrive consecutively in a serial manner. The numbers of switches that are connected in a fabric topology are increasing. This also increases delivery of out of order frames, i.e., frames that are not received in-order. The current SCSI_FCP standard and SAN systems do not handle out of order frames efficiently. If a frame is out of order or dropped, the entire Exchange operation is performed again. This causes delay and latency.
Therefore, what is required is a system and method for efficiently handling out of order frames in a SCSI_FCP environment.